Influence of Carotid Baroreceptors and Vasoactive Drugs on Systemic Vascular Volume and Venous Distensibility

THE IMPORTANCE of the carotid and aortic baroreceptors in the reflex control of heart rate, peripheral vascular resistance, and myocardial contractility is well appreciated. It has long been suspected that an important additional factor in circulatory regulation is the distensibility and capacity of the venous bed." The multiple circulatory effects of the baroreceptors have made precise study of their influence on total systemic vascular volume and on venous distensibility difficult. Nevertheless, several investigators have demonstrated changes in the capacity of isolated venous segments resulting from activation of carotid sinus receptors," and other studies have suggested that such reflexes may affect the entire venous bed.The present investigation was carried out in an experimental preparation from which the heart and pulmonary circulation were excluded by means of total body perfusion. First, it was designed to permit study of the effects of alterations in pressure in the isolated carotid sinuses on the volume of the total systemic vascular bed and on venous return. Second, a method was devised for the determination of pressure-volume relationships in the systemic veins by briefly occluding the total venous outflow while inflow was held constant. This technique was utilized to study the effects of carotid sinus activation on the distensibility of the venous bed. In the past, similar approaches to the measurement of venous distensibility have included only portions of the total venous bed.' Finally, a comparison was made between the reflexly induced changes in: ( l j systemic vascular vol-